Recovery of caprolactam from nylon 6 scrap (in other words, nylon 6 polymer that is substantially free of non-nylon 6 materials) has been practiced for at least twenty years. In general, nylon 6 is depolymerized by heating at elevated temperatures, usually in the presence of a catalyst and/or steam. The caprolactam produced is removed as a vapor stream. An extensive review of the field has been given by L. A. Dmitrieva et al, Fibre Chemistry, Vol. 17, No. 4, Mar. 1986, pp 229-241. Depolymerization of hydrolyzable polymers that are produced as scrap during the manufacture of fiber, chip, film or molded articles is also described in U.S. Pat. No. 4,605,762 to Mandoki. The process includes introducing the polymeric scrap into a hydrolyzer at a temperature of 200.degree. to 300.degree. C. and a pressure of at least 15 atmospheres, wherein high pressure steam is introduced into the lower portion of the hydrolyzer below the polymeric scrap. An aqueous solution of the products of the hydrolysis reaction is withdrawn from an upper portion of the hydrolyzer.
In the case of multi-component mixtures or composites that contain nylon 6 as one component, however, recovery of caprolactam is complicated by the presence of the other components. These other components and/or their decomposition products generated under conventional nylon 6 depolymerization conditions interfere with the isolation of caprolactam of adequate purity, thus necessitating expensive additional purification steps.
It would be particularly beneficial if an inexpensive method could be developed for the recovery of caprolactam from multi-component composites or materials that include nylon 6, such as carpets. The prospect of recycling such material presents a tremendous opportunity to reduce landfill usage and the costs of disposal, as well as an opportunity to reuse natural resources.
Carpets include a face fiber that is adhered to a backing (support) material which may include jute, polypropylene, latex (such as a styrene-butadiene rubber (SBR)) and a variety of inorganic materials such as calcium carbonate, clay or hydrated alumina fillers. Nylon 6 is often used for the face fiber. Typically, the face fiber constitutes only 20-50% by weight of the carpet, the rest of it consisting of the backing materials. In addition, the fiber contains dyes, soil repellants, stabilizers and other compounds added during fiber and/or carpet manufacture. Waste carpet may also contain a host of other impurities, which will collectively be referred to herein as "dirt".
These non-nylon 6 components interfere with caprolactam recovery. For example, one of the most difficult problems is that alkaline components, such as the calcium carbonate filler, neutralize the acidic catalysts, such as phosphoric acid, that are conventionally used to promote nylon 6 depolymerization, thus requiring the use of increased amounts of catalyst. Another problem is that polypropylene and latex partially decompose to a mixture of hydrocarbons that co-distill with caprolactam. The remaining, partially decomposed, non-distilled portion, along with the filler and other inorganic components, renders the reaction mixture very viscous and difficult to process in conventional equipment.
U.S. Pat. No. 5,216,149 to Evans et al. attempts to solve the general problem of reclaiming useable materials from multi-component plastic waste by using "fast pyrolysis in a carrier gas" in the presence of a catalyst. Example 1 of the patent is directed to the recovery of caprolactam from a waste stream containing nylon 6. The reaction is conducted at a temperature sufficiently high to cause pyrolysis of nylon 6 but not of the other components. However, in the exemplified experiments for which data is reported the only other component used in the waste stream mixture with nylon 6 was polypropylene.
U.S. Pat. No. 5,169,870 to Corbin et al. also describes a method for attempting to reclaim caprolactam from carpets that contain nylon 6 face fibers. The method includes mechanically separating a portion of the non-nylon 6 components and catalytically depolymerizing the resultant enriched nylon 6 fraction in the presence of superheated steam. The crude yield of caprolactam given in Example 1 of the patent was 56%; steam and 85% phosphoric acid were used respectively at the rate of 33 and 0.55 parts per part of crude caprolactam produced. It is stated in the patent that the initial mechanical separation step is not necessarily required and in Example 3 of the patent a carpet was depolymerized without prior mechanical separation and steam and 85% phosphoric acid were used respectively at the rate of 51 and 0.30 parts per part of crude caprolactam produced.
In Czechoslovakian Pat. No. 143,502 to Petru et al. there is described a process for the recovery of caprolactam from a waste material that includes nylon 6 and a non-nylon 6 component, such as in tires, laminated plastic sheets or textiles from mixed fibers. The waste material is heated under pressure in water at the melting point of nylon 6. The extracted nylon 6 is subsequently subjected to a separate step of thermal depolymerization, with or without prior separation from water or another solvent.
Smith, S., in the Journal of Polymer Science, Vol. 30, pp. 459-478 (1958), describes the depolymerization of nylon 6 in the presence of water in a sealed system as being an equilibrium reaction. This article reports both the rate and the equilibrium composition at 230.degree. and 270.degree. C.
A need still exists, however, for an efficient process for recovery of caprolactam from multi-component materials that include nylon 6.